A nerve is an enclosed, cable-like bundle of axons. A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Each nerve contains many axons. Each axon is surrounded by a layer of connective tissue called the endoneurium. The axons are bundled together into groups called fascicles, and each fascicle is wrapped in a layer of connective tissue called the perineurium. Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium.
When a nerve axon is severed, the end still attached to the cell body is labeled the proximal segment, while the other end is called the distal segment. Neuroregeneration in the peripheral nervous system (PNS) occurs by axonal sprouts forming at the proximal stump and growing until they reach the distal stump. The growth of the sprouts are governed by chemotactic factors secreted from Schwann cells (neurolemmocytes). The proximal axons are able to regrow as long as the cell body is intact, and they have made contact with the Schwann cells in the endoneurial channel. Human axon growth rates can reach 2 mm/day in small nerves and 5 mm/day in large nerves. The distal segment, however, experiences Wallerian degeneration within hours of the injury; the axons and myelin degenerate, but the endoneurium remains. In the later stages of regeneration the remaining endoneurial tube directs axon growth back to the correct targets. During Wallerian degeneration, Schwann cells grow in ordered columns along the endoneurial tube, creating a band of Bungner (boB) that protects and preserves the endoneurial channel. Also, macrophages and Schwann cells release neurotrophic factors that enhance re-growth.